Lead-acid batteries and cells have been known for a substantially long period of time and have been employed commercially in a relatively wide variety of applications. Such applications have ranged from starting, lighting and ignition for automobiles, trucks and other vehicles (often termed "SLI batteries") to marine and golf cart applications and to various stationary and motive power source applications (sometimes termed "industrial battery" applications).
The lead-acid electrochemical system provides a reliable energy source which is capable of being manufactured in automated production while providing acceptable quality. At this time, battery containers are generally manufactured in large volumes as injection molded plastic parts. As the battery container includes five of the six sides of the exterior of the battery, this component is largely responsible for the final dimensions of the battery, as well as its cosmetic appearance. Beyond the appearance of the battery, the dimensions of the upper opening of the container must be sufficiently precise to permit a seal between the container and the lid of the battery in order to ensure proper operation and prevent leakage.
During use, however, lead-acid batteries may develop or be exposed to extremely high operating temperatures and pressures. The electrochemical reactions within the cells of a lead-acid battery, particularly in a recombinant sealed battery, result in the development of high pressures, as well as high temperatures. While the exact parameters reached will vary based upon the particular battery design, the internal pressure of a battery, for example, may reach on the order of three to six pounds per square inch (3-6 p.s.i.), while the temperature may reach over 200.degree. F.
These high pressures and temperatures within the battery may cause the battery container to deflect and distort. This deflection may be restrained along the side walls of the container inasmuch as the partitions between the cells extend crosswise through the battery from side wall to side wall. Accordingly, the bulk of such deflection occurs on the end walls of the container where there are no interior partitions to restrain the deflection. In tests of a Group 27 battery of the assignee of the present invention, the end wall of the container having vertical ribbing was measured to deflect 0.085 inch at 1 p.s.i., 0.236 inch at 3 p.s.i., and 0.342 inch at 5 p.s.i.
This deflection may adversely affect the performance of the battery as well as the cosmetic appearance. As the end walls deflect, the cells expand, allowing the plates to separate and pull apart. This reduction in cell compression results in a corresponding reduction in battery performance. Further, the deflection of the end walls increases the effective length of the battery and decreases the overall attractiveness thereof. It has further been observed that in severe cases, the plastic container may crack at points of high deflection and stress, resulting in leaks.
This problem may be exacerbated by the environmental conditions of the battery. For example, current vehicles, particularly automobiles, emphasize aerodynamic styling and are equipped with a variety of driver comfort features and safety devices. These features have resulted in such vehicles operating in many situations with very high underhood engine temperatures. The battery may be located in the front of the underhood compartment, where there is little air movement, or where the engine fan blows hot air directly onto the battery. Accordingly, during stop-and-go driving, or while the engine of the vehicles is idling, there is typically very little air or wind movement, causing the underhood air temperatures to often exceed 200.degree. F. in some parts of the United States. Thus, these increased temperatures may further contribute to distortion of the battery container during operation.
In the early part of the twentieth century and up to the sixties, battery containers were constructed of molded hard rubber, sometimes using coal as a filler. On occasion, the molded rubber container was surrounded by a wooden box in order to permit easy handling or restrain the walls of the container. Further, because the container was made of molded rubber, it could readily be molded to a thicker dimension in order to minimize any deflection thereof. Recombinant sealed batteries, however, were not developed and did not come into common use until the late 1970's and early 1980's. Accordingly, the high internal pressures associated therewith were not typically even a problem with batteries which utilized molded hard rubber containers prior to the advent of the plastic battery container. Accordingly, deflection of the end walls due to the batteries developing high internal pressures or temperatures during use was not typically a design consideration with molded rubber containers. Molded rubber containers also had certain disadvantages. Due to the thick, dense walls of the container, they are relatively heavy. Additionally, such containers were relatively fragile.
While molded plastic containers are advantageous in view of size and weight, molding of plastic presents certain processing and design limitations, particularly in recombinant sealed batteries. In particular, molded plastic components exhibit different shrinkage factors depending upon the geometry and part thickness. As a result, and contrary to the design of molded hard rubber containers, the thickness of the end walls of a battery container may not be disparately greater than the thickness of the side walls or the partitions between the cell of the container without incumbent molding difficulties. Accordingly, battery designers have sometimes incorporated vertical and horizontal ribbing in the battery container in order to reduce container wall deflection. This design feature, however, has met with limited success.